1. Field of the Invention
The present invention relates to integrated circuit packaging, and more specifically to a leadframe structure and method of using same which provides an integrated leadframe and exposed bezel portion for receiving a user's fingertip or the like.
2. Description of the Prior Art
The present invention lies at the crossroads of two different technologies. The first of these is integrated circuit (IC) packaging. The second is silicon fingerprint sensors. We look first at IC packaging. Historically, the number of devices formed on an IC die has increased dramatically from year to year. Advancements in both the design and layout of devices carried by the IC die as well as in designs for making external connections to those devices have supported the increases in device count. A typical IC assembly includes the die, often a body of a semiconducting material such as silicon, having interconnected electronic elements such as transistors, resistors, capacitors, interconnections, etc. formed thereon. The die are typically quite small, with correspondingly small contact pads, necessitating use of a secondary structure to make practical electrical connections between the die and a printed circuit board (PCB) or other body to which the IC is attached for use. Such secondary structures include leadframes, chip carriers, and the like. In common applications, an IC die is physically bonded to a leadframe, and fine wires make the electrical interconnections between the bonding pads of the IC die and the bonding leads of the leadframe. The leadframe in turn presents pads or pins which make the final electrical connection to the next level PCB or the like.
The IC die, connection wires, and bonding leads of the leadframe are typically encapsulated in a non-conductive structure or encapsulation, such as resin or plastic. In certain embodiments, a portion of the leadframe extends beyond the extent of the encapsulation for the external connection points to the leadframe and thereby to the IC. In other embodiments the leadframe is etched to have at least two different thicknesses. The regions having the lesser of the thicknesses become fully encapsulated in the encapsulation material, while the regions having the greater thickness protrude beyond the encapsulation material, providing external contact pads.
The encapsulation provides protection from both mechanical (e.g., impact and scratch) and electrical (e.g., electrostatic discharge) damage. The encapsulation material is nonconductive, providing internal electrical isolation as well as isolating the die therein from unwanted external electrical contact.
Manufacturing the encapsulated die and leadframe combination is most commonly accomplished by placing the die, pre-mounted to a die pad portion of the leadframe, within the cavity of a mold, and injecting encapsulation material into the cavity. By sizing and properly engineering the mold, the encapsulation material is applied to a desired thickness and in desired regions around the leadframe and IC die.
We turn next to silicon fingerprint sensors. Devices designed for sensing the pattern of a fingerprint fall into several categories based on the type of sensor they employ, such as optical, thermal, capacitive, and so on. Within these categories, some require a user's finger be in direct contact with a portion of the sensor (or a coating applied thereover) such as in the case of capacitive sensors, while others require that the finger be positioned spaced apart from the sensor surface (though often in direct contact with another surface such as a glass or plastic optical platen or lens surface), such as in the case of optical sensors. We are primarily concerned in this disclosure with the category of direct contact sensor devices, although some aspects of the present invention may be usefully applied to non-direct-contact sensor devices, as will be appreciated by one skilled in the art.
In one example of a direct contact silicon fingerprint sensor device, a capacitor plate is formed just below the surface of an IC sensor die. Typical IC fabrication techniques are employed to form the sensor die. A fingertip is placed on the surface of the sensor die. The skin surface at the ridges of the fingerprint will therefore be located closer to the capacitor plate than the skin surface at the valleys. The skin surface is established as one plate of a capacitor, and the buried plate as another. Thus, the distance between the capacitor plates varies as between ridges and valleys of the fingerprint pattern. This variation in distance results in a variation in capacitance, which may be measured for regions (i.e., pixels) of the fingertip and used to create a 3-dimensional representation of the fingerprint contours and pattern (where distance corresponds to pixel grayscale value).
Direct contact capacitive sensor (and other direct-contact sensor technologies such as thermal) devices require that the sensed portion of a user (e.g., the fingertip) be positioned very close to, if not touching the sensor device. For this reason, sensor devices for direct contact fingerprint sensing are often packaged in encapsulation material such that a portion of the surface of or over the IC sensor die is exposed (i.e., the mold structure is such that the encapsulation material is prevented from forming over a sensing portion of the IC die). In some applications one or more thin protective coatings such as polyamide or epoxy are applied to the sensor and or IC surface at the wafer level (prior to packaging), in other applications such thin protective coatings can be applied within the packaging process itself (covering the silicon die with a thin layer of encapsulation material to form an outermost sensing surface), and in still other applications no additional protective coating (beyond the standard passivation layer typically applied as one of the final steps of silicon wafer manufacturing) is used and the silicon sensor surface (e.g., silicon nitride passivation layer) is actually exposed for contact. Whether thinly covered with additional protective layers beyond the one typical passivation-based protective layer or not, and whether any such protective layers are in place prior to encapsulation or formed as part of the encapsulation process, such devices are referred to herein as exposed die sensor packages. The sensing surface is the uppermost surface of such exposed die sensor packages.
There are two further subcategories of direct-contact silicon fingerprint sensor devices—area sensors and strip sensors. An area sensor is essentially a two-dimensional array of sensor pixels over which a user places a finger. Through a raster or similar scanning process, the array of sensor pixels are sequentially activated, sensing takes place, and the results read out for processing. No motion of the finger is required. Indeed, the finger must be kept relatively motionless during the sensing phase to obtain undistorted results. A strip sensor is essentially a one-dimensional array (or an array with a much greater width than length), over which a user swipes a finger. The sensor may typically be as wide as the width of an average fingertip. For example, such a sensor may be in the range of 5-10 mm in width, and typically measures 0.1-0.4 mm or 2-8 sensor rows in length. As the finger passes over the sensor, sensing takes place for a “strip” of the fingerprint equal in length to the length of the sensor array (which is again many fewer pixels than the width). Each image is an accurate representation of the structure of a small essentially 1-dimensional strip of the overall 2-dimensional fingerprint pattern. A complete 2-dimensional image of the fingerprint pattern is then composed by software from analysis and normalization of the data from the individual essentially 1-dimensional strips. A very compact sensor is thereby provided, which finds use in many devices such as portable (notebook) computers, telephones (such as cell phones), personal digital assistants (PDAs), etc.
There are a number of existing techniques for producing an exposed die sensor package. Each typically include disposing the die and a contact structure within a mold body, and injecting resin or plastic into the mold to encapsulate the die. According to one technique, disclosed in U.S. Pat. No. 6,686,227, which is incorporated herein by reference, a die may be mounted to an insulative substrate (such as a fiberglass panel), with bonding wires making electrical connection between the die and substrate. The die and substrate are then placed in a mold body, such that the mold body clamps the substrate to hold the structure in place. A seal, located either on the mold or on the substrate then serves to block any introduced encapsulation material injected into the mold from being applied to the region of the die which is desirably to be exposed. Upon removal from the mold body, the die and substrate are encapsulated in the protective encapsulation material, with an exposed portion of the die provided for the sensing operation.
Another technique, disclosed in U.S. Pat. No. 5,862,248, which is incorporated herein by reference, provides discrete leads in a molding process resulting in an exposed die with a dual-in-line package (DIP) type lead arrangement. According to this technique, a die is attached to a leadframe and pads on the die are electrically connected to the leadframe by wire bonds. A patterned region of removable material is formed over the die where it is desired that a portion of the die be exposed after molding. The die and leadframe are then positioned within the cavity of a mold, and encapsulation material injected into the cavity. The removable material is then removed to produce an encapsulated structure with an exposed region. That is, a structure is produced which is entirely encapsulated in encapsulation material apart from the location blocked by the removable material. Accordingly, the integrally molded structure includes encapsulation material beneath the leadframe (i.e., on the side of the leadframe opposite that to which the die is attached).
In an alternate embodiment of capacitive sensor devices, such as that disclosed in U.S. Pat. No. 6,512,381, which is incorporated herein by reference, a varying voltage electrically drives the fingertip being sensed during the sensing process. The variable two-plate sense capacitor previously described can also be used to provide a variable input charge into more complex capacitive sensor systems based on the sensing of fringing field interference caused by the presence of fingerprint ridges. With the addition of an external electrode to electrically drive the finger with a varying voltage so that the presence or absence of fingerprint ridges act as a variable charge transfer input capacitor to complement the effect the ridges of the fingerprint have in acting to interfere with the fringing field of the sensor capacitor, the sensitivity of the sensor is thereby greatly improved.
In order to drive the user's finger with the desired varying voltage, the finger must be in electrical contact with a voltage source. According to one embodiment, this contact is made by providing a metal bezel around part or all of the perimeter of the sensor. As the user applies a finger to the sensor surface, either by placement on an area sensor or in the swiping motion over a strip sensor, the finger in put into direct contact with the bezel (or in contact with a conductive coating applied over the bezel in manufacturing). The bezel then serves as the contact with the finger for transferring the charge from the finger to the input capacitor of the sensor apparatus.
Traditionally, the metal bezel, the leadframe, and the sensor IC have each been separate elements, brought together in the process of assembling or packaging the sensor apparatus. However, as in the general art of IC production, there is significant, ongoing commercial pressure to reduce cost, number of components, and number and complexity of manufacturing steps, and size of the completed structure. The present invention focuses on the bezel and leadframe to reduce cost, complexity, and size, of the sensor apparatus, simplifying its assembly, etc.